A set of Fresnel technology mirrors, hereafter called FTM, is made up of planar, longilineal rectangular mirrors. These longilineal mirrors are assembled to form strips of mirrors. For example, for an FTM assembly X meters long and 8 m wide, the strips of planar mirrors may have a length of X meters and a width from 5 to several 20 cm or more.
Each of the strips of planar mirrors can be oriented around its respective median axis, called large axis, which is generally horizontal and most of the time oriented in an East/West direction to give it an appropriate orientation to reflect the incident rays of the sun toward a concentrator element, such as a concentrator tube in which a coolant circulates.
The solar sensor generally has a vertical plane of symmetry passing through a median axis of the large axes of the planar mirrors. This plane of symmetry virtually separates the FTMs into two parts.
A Fresnel mirror solar power station also includes a secondary mirror, as an extension of the primary mirrors made up of the strips of planar mirrors. The secondary mirror generally has a mono or bicylindro-parabolic shape, is situated above the concentrator tube, and serves to reflect the portion of the radiation reflected by the FTMs that has not directly reached the concentrator tube toward the concentrator tube.
The secondary mirror is necessary in cases where the concentrator tube has a diameter smaller than the width of a primary planar mirror strip. However, the use of the secondary mirror slightly decreases the thermal flow reaching the concentrator tube, and therefore the heating thereof.
The decision not to install secondary mirrors means:                determining a width of the strips of planar mirrors smaller than the diameter of the tube (or of the tube provided with fins, as found on the market for vacuum tube heat sensors),        particularly precise subjugation of the strips of planar mirrors.        
Since the installation of the concentrator tube is stationary, it is in theory possible to coordinate the movement of each of the strips of planar mirrors making up the FTMs relative to the position of the sun through a single actuator connected to each of the strips of planar mirrors, for example by a specific rod. The control and command device dedicated to following the sun is therefore very simple and inexpensive.
It is known from document U.S. Pat. No. 4,229,076 to arrange the set of strips of planar mirrors of the FTMs on a support that can be oriented along a vertical axis so that the median axis of the large axes of the planar mirrors follows the azimuth of the sun.
In this way, one substantially decreases the length of the ineffective area of the concentrator tube, i.e. the area of the tube not reached and therefore not heated by the rays of the sun reflected by the strips of planar mirrors, and on the other hand the length of the lost area, i.e. the area of the space located in the extension of the tube and that is passed through by the rays of sunlight reflected by the strips of planar mirrors without the latter contributing to heating the concentrator tube.
The solar power stations with Fresnel mirrors are satisfactory for low altitudes below 30° North or South in that they make it possible to heat a coolant circulating in the concentrator tube to effective temperatures to produce hot water or steam directly or through a heat exchanger. The steam then actuates a turbine, which produces electricity.
This form of energy production has several advantages, including using a renewable energy source that is free and nonpolluting. Furthermore, using FTMs makes it possible to produce, at a low cost, a large surface to collect the rays of the sun. Furthermore, the planar surface of the mirrors is relatively easy to clean compared to surfaces of curved mirrors.
In fact, the planar mirrors are less expensive to manufacture and maintain than the parabolic or cylindro-parabolic mirrors used for similar applications; their installation requires a relatively light structure and their orientation, around a single axis, requires relatively simple means to implement. It is therefore easy to produce and maintain a surface to collect the rays of the sun that has large dimensions and a lower cost.
These Fresnel mirror solar power stations also have the advantage of being able to be installed easily in urban settings. For example, it is possible to install FTMs at 50 cm from the ground on building terraces without particular inconvenience due to their visibility or the risk of flight under the action of the wind, that risk being more significant for the parabolic or cylindro-parabolic mirrors, which can generate pressure gradients between the front and rear surfaces thereof due to their curvature.
The FTMs are installed on stationary structures, their large axes being oriented most of the time along an East/West axis allowing them to collect maximum radiation during the relative movement of the sun in the sky.
However, since the sun does not rise strictly in the East and does not set strictly in the West, and for relatively low heights of the sun on the horizon, i.e. primarily close to the sunrise and sunset positions, and for one of the two parts of the FTMs previously defined, the end of the first strip of planar mirrors positioned first relative to the sun and oriented in rotation around its large axis to reflect the radiation thereof, conceals part of a second strip of planar mirrors adjacent to the first and further from the sun by creating a shadowed zone thereon.
This concealment of the incident radiation is maximal two times during the day: once in the morning and once in the evening, the shade area on a strip of mirrors being situated sometimes on one edge, sometimes on the other for these two periods of the day.
The surface occupied by the shadow zone on the second strip of mirror is not active, as it does not contribute to heating the concentrator tube.
Likewise, for the same relatively low heights of the sun, and for the second part of the FTMs previously defined, the radiation reflected on one end of a first strip of planar mirrors oriented to reflect the incident radiation coming from the sun on the concentrator tube can be concealed by an end of a second, adjacent strip of planar mirrors, which is also oriented to reflect the incident rays of the sun on the concentrator tube, while creating a lost area on the first strip of mirrors.
This concealment of the reflected radiation is maximal two times during the day: once in the morning and once in the evening, the lost area on a strip of mirrors being situated sometimes on one edge, sometimes on the other edge for these two periods of the day.
The surface occupied by the lost area on the first strip of mirrors is also not active, as it does not contribute to heating the concentrator tube.
The presence of these surfaces reduces the quantity of reflected radiation contributing to heating the concentrator tube. The area of the surfaces depends on:                the width of the strips of mirrors,        the space between the strips of mirrors to allow them sufficient play,        the angle of incidence of the incident ray and therefore the height of the sun,        the position of the concentrator tube, in particular its distance from the strips of mirrors.        
However, the main drawback of the Fresnel mirror solar power stations as they are described in the state of the art lies in the fact that they must be installed in locations with a relatively low latitude to have satisfactory yields.
The first reason for this necessity is factual: in these low-latitude regions, i.e. close to the equator, the annual average sunshine is higher than in middle-latitude regions, such as France, for example, and high-latitude regions.
The second reason, which is more technical, is related to the fact that the solar rays at these latitudes are relatively low. Thus, for a location situated at 45° North, the height of the sun at noon in winter will be approximately 23°, and in summer approximately 68°.
Under these conditions, the Fresnel mirror solar power station operates with angles of incidence reducing the performance of the power station.
It has been observed that the rays of the sun striking the strips of planar mirrors with an angle of incidence of 23° generate radiation reflected in the longitudinal direction of the tube reaching the concentrator tube quite far from their point of origin situated on the strips of planar mirrors, generating an ineffective area on the concentrator tube in which this type of reflected radiation does not contribute to the thermal flow.
Likewise, part of the reflected radiation does not reach the concentrator tube, and generates a lost area in which this type of reflected radiation does not contribute to the thermal flow.
These two lacks of contribution reduce the potential heat production of the solar sensor intended to heat the coolant located inside the concentrator tube.
As previously seen for document U.S. Pat. No. 4,229,076, this drawback may be resolved in part by installing FTMs on the support pivoting around a vertical axis.
However, this implementation requires having a free surface corresponding to a disk portion with a diameter equal to the length of the strips of mirrors to allow the FTMs to pivot on a vertical axis.
An economic analysis shows that the solution is not compatible with the property surface areas made available on roofs or building terraces.
Furthermore, the effectiveness of such solar sensors is directly related to whether the solar disk is or is not visible. Such sensors are not effective when the disk of the sun is not visible, for example in cloudy weather, which occurs frequently in the middle-latitude areas.
However, the solar power is characterized by the direct rays of the visible disk and the radiation diffused in random directions.
For an annual period, in given geographic locations, the power ratio between the direct radiation and the diffuse radiation may approach 1.
In these cases, the annual direct radiation power is of the same order as the annual power of the diffuse radiation per unit of horizontal surface.
The diffuse radiation should also be used in a solar sensor.
Lastly, the use of such a solar sensor should not be limited solely to heat production.